SPIN TRANSFER TORQUE TUNNELING MAGNETORESISTIVE DEVICE HAVING A LAMINATED FREE LAYER WITH PERPENDICULAR MAGNETIC ANISOTROPY
A spin transfer torque magnetic junction includes a magnetic reference layer structure with magnetic anisotropy perpendicular to a substrate plane. A laminated magnetic free layer comprises at least three sublayers (e.g. sub-layers of CoFeB, CoPt, FePt, or CoPd) having magnetic anisotropy perpendicular to the substrate plane. Each such sublayer is separated from an adjacent one by a dusting layer (e.g. tantalum). An insulative barrier layer (e.g. MgO) is disposed between the laminated free layer and the magnetic reference layer structure. The spin transfer torque magnetic junction includes conductive base and top electrodes, and a current polarizing structure that has magnetic anisotropy parallel to the substrate plane. In certain embodiments, the current polarizing structure may also include a non-magnetic spacer layer (e.g. MgO, copper, etc).
This application is a continuation of U.S. patent application Ser. No. 14/563,721 to Shaoping Li, entitled “Spin Transfer Torque Tunneling Magnetoresistive Device Having a Laminated Free Layer with Perpendicular Magnetic Anisotropy,” filed 2014 Dec. 8, pending.
BACKGROUNDInformation storage devices are manufactured in high volume and widely used to store and/or retrieve data in computers and other consumer electronics devices. Information storage devices may be classified as volatile or non-volatile, depending upon whether their memory content is maintained when the information storage device is not powered. Examples of non-volatile information storage devices include magnetic hard disk drives and magnetic random access memory (MRAM) devices, either of which may utilize a magnetoresistive tunnel junction (MTJ) as part of information storage or retrieval operations. Specifically, whereas volatile random access memory (RAM) devices typically store data as electric charge, MRAM devices may store data in MTJs that maintain memory content even when the memory device is not powered.
Generally, each MTJ includes a reference layer that has a magnetic orientation that is pinned or fixed, and a free layer having a magnetic orientation that can be changed by an external magnetic field (e.g. from an adjacent disk or generated by a programming current). The MTJ is in a low resistance state when the free layer magnetic orientation is parallel to that of the reference layer, and in a high resistance state when the free layer magnetic orientation is anti-parallel to that of the reference layer. If the external magnetic field and/or programming current required to switch a desired MTJ between high and low resistance states (with acceptable switching speed) is too great, or if the MTJs are arranged too closely together, then one or more adjacent MTJs may undesirably be affected or inadvertently switched.
There have been many patented variations and improvements to MTJs in recent years, some of which help mitigate the foregoing problem to allow for more reliable operation when the MTJs are arranged in close proximity to each other. For example, a spin transfer torque magnetic random access memory (STT-MRAM) has been investigated, in which each MTJ is switched (i.e. “programmed”) by an application of spin polarized current through the MTJ. The STT-MRAM promises significant advantages over magnetic-field-switched MRAM, which has been recently commercialized. For example, STT-MRAM can be scaled to a smaller size while maintaining the programmability of individual MTJs (without inadvertently and undesirably affecting the programming of adjacent MTJs). Moreover, STT-MRAM can be programmed with lesser programming currents, which reduces power consumption and associated requirements for heat dissipation.
However, one of the challenges for implementing STT-MRAM is minimizing the programming current required to quickly switch the magnetic orientation of the free layer in the MTJ, while maintaining high thermal stability of the recorded data. Reduced programming current may enable the use of smaller memory cell transistors, which can substantially reduce the overall size of the incorporating memory device. A reduced programming current requirement also corresponds to reduced voltages across the MTJs during writing and sensing, which may decrease the probability of tunneling barrier degradation and breakdown, and thereby improve the endurance and reliability of the incorporating memory device.
Hence, there is an ongoing need in the art for an improved MTJ that can quickly and reliably switch with acceptable thermal stability using a reduced programming current, and that is suitable for high volume manufacture and widespread durable use in reduced-scale data storage devices.
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When the MRAM cell 50 is selected to be programmed (in part by selective operation of the transistor 54 by word line 58), the read/write circuitry 62 applies a programming current through the MTJ stack 52 via the bit line 56, the source line 60, and the transistor 54. For example, the read/write circuitry 62 may apply a write voltage between the bit line 56 and the source line 60, with the polarity of such write voltage determining the remnant magnetization of the free layer in the MTJ stack 52. Specifically, the MTJ stack 52 may operate on a spin transfer torque principle, in which case electrons of the programming current become spin-polarized as the electrons pass through a spin filter layer of the MTJ stack 52. In that case, spin-polarized electrons of the programming current may exert a torque on the free layer of the MTJ stack 52, which can switch the polarity of the free layer during the writing operation.
To read the MRAM cell 50, a sensing current is used to detect the resistance state of the MTJ stack 52, with the sensing current being less than the programming current (so that sensing does not switch the free layer polarity in the MTJ stack 52). The read/write circuitry 62 may apply the sensing current to the MTJ stack 52 via the bit line 56, the source line 60, and the transistor 54. The programmed state of the MRAM cell 50 is sensed according to the resistance across the MTJ stack 52, which may be determined by the voltage difference between the bit line 56 and the source line 60. The voltage difference may optionally be compared to a reference 64 by a sense amplifier 66.
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The presently disclosed structure and composition of the laminated free layer 240 of the embodiment of
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The aforedescribed arrangement and composition of the spin transfer torque MTJ 200 may enable perpendicular anisotropy of the magnetic laminated free layer 240, without the need to overly increase the iron content of the sublayers 242, 244, 246 or excessively decrease their thickness (to promote perpendicular anisotropy). The sublayers 242, 244, 246 may undesirably become super-paramagnetic and therefore thermally unstable if they were instead required to be excessively thin (where the memory cell size is small enough for practical use in modern data storage). Hence, the aforedescribed arrangement and composition of the spin transfer torque MTJ 200 may beneficially enable perpendicular anisotropy with thicker free layer laminates, and thereby increase switching speed and/or reduce the risk of thermal instability.
The aforedescribed sequence and order of deposition of the layers of the spin transfer torque MTJ 200 is not the only possible sequence or order of deposition according to the present invention. For example,
The spin transfer torque MTJ 300 of
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The presently disclosed structure and composition of the laminated free layer 340 of the embodiment of
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In certain embodiments, the current polarizing structure 360 is preferably spaced from the laminated magnetic free layer 340 by a MgO spacer layer 366 that is adjacent to the laminated magnetic free layer 340. In such embodiments, the MgO spacer layer 366 preferably has a thickness in the range of 2 to 8 Angstroms, which may provide a resistance area product (“RA”) in the range 0.2 to 0.8 Ω·μm2—preferably substantially less than that of the insulative barrier layer 330.
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The magnetic reference layer structure 310 may also include a pair of antiferromagnetically coupled pinning layers 314, 316 separated by a thin non-magnetic dusting layer 315 (e.g. ruthenium). The magnetic reference layer structure 310 may optionally further include an outer layer 318 (optionally comprising an alloy of manganese, such as PtMn, IrMn, NiMn, FeMn).
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The aforedescribed arrangement and composition of the spin transfer torque MTJ 300 may enable perpendicular anisotropy of the magnetic laminated free layer 340, without the need to overly increase the iron content of the sublayers 342, 344, 346 or excessively decrease their thickness (to promote perpendicular anisotropy). The sublayers 342, 344, 346 may undesirably become super-paramagnetic and therefore thermally unstable if they were instead required to be excessively thin (where the memory cell size is small enough for practical use in modern data storage). Hence, the aforedescribed arrangement and composition of the spin transfer torque MTJ 300 may beneficially enable perpendicular anisotropy with thicker free layer laminates, and thereby increase switching speed and/or reduce the risk of thermal instability.
In the foregoing specification, the invention is described with reference to specific exemplary embodiments, but those skilled in the art will recognize that the invention is not limited to those. It is contemplated that various features and aspects of the invention may be used individually or jointly and possibly in a different environment or application. The specification and drawings are, accordingly, to be regarded as illustrative and exemplary rather than restrictive. For example, the word “preferably,” and the phrase “preferably but not necessarily,” are used synonymously herein to consistently include the meaning of “not necessarily” or optionally. The drawings are not necessarily to scale. “Comprising,” “including,” and “having,” are intended to be open-ended terms.
Claims
1. A spin transfer torque magnetic junction comprising:
- an electrically conductive base electrode layer defining a substrate plane;
- a magnetic reference layer structure disposed on the base electrode layer and having magnetic anisotropy perpendicular to the substrate plane;
- an insulative barrier layer disposed on the laminated magnetic reference layer structure;
- a laminated magnetic free layer disposed on the insulative barrier layer and having magnetic anisotropy perpendicular to the substrate plane,
- a current polarizing structure disposed on the laminated magnetic free layer and having magnetic anisotropy parallel to the substrate plane;
- an electrically conductive top electrode layer disposed on the current polarizing structure;
- wherein the laminated magnetic free layer comprises at least three sublayers of CoFeB, each being separated from an adjacent CoFeB sublayer by one of a plurality of dusting layers.
2. The spin transfer torque magnetic junction of claim 1 wherein the laminated magnetic free layer comprises no more than four sublayers of CoFeB.
3. The spin transfer torque magnetic junction of claim 1 wherein each of the plurality of dusting layers comprises tantalum with a thickness in the range of 1 Angstroms to 7 Angstroms.
4. The spin transfer torque magnetic junction of claim 1 wherein the current polarizing structure comprises a CoPd layer that is adjacent to the top electrode layer, and a MgO spacer layer that is adjacent to the laminated magnetic free layer.
5. The spin transfer torque magnetic junction of claim 1 wherein the magnetic reference layer structure comprises a pinned layer comprising CoFeB that is disposed adjacent the insulative barrier layer, and a pair of antiferromagnetically coupled pinning layers, each having magnetic anisotropy perpendicular to the substrate plane.
6. The spin transfer torque magnetic junction of claim 5 wherein the magnetic reference layer structure further comprises an outer layer comprising an alloy of manganese that is disposed adjacent to the base electrode layer.
7. The spin transfer torque magnetic junction of claim 1 wherein the insulative barrier layer comprises MgO having a thickness in the range of 8 Angstroms to 20 Angstroms.
8. A spin transfer torque magnetic junction comprising:
- an electrically conductive base electrode layer defining a substrate plane;
- a current polarizing structure disposed on the base electrode layer and having magnetic anisotropy parallel to the substrate plane;
- a laminated magnetic free layer disposed on the current polarizing structure and having magnetic anisotropy perpendicular to the substrate plane;
- an insulative barrier layer disposed on the laminated magnetic free layer;
- a magnetic reference layer structure disposed on the insulative barrier layer and having magnetic anisotropy perpendicular to the substrate plane;
- an electrically conductive top electrode disposed on the magnetic reference layer structure;
- wherein the laminated magnetic free layer comprises at least three sublayers of CoFeB, each being separated from an adjacent CoFeB sublayer by one of a plurality of dusting layers.
9. The spin transfer torque magnetic junction of claim 8 wherein each of the plurality of dusting layers comprises tantalum with a thickness in the range of 1 Angstroms to 7 Angstroms.
10. The spin transfer torque magnetic junction of claim 8 wherein the current polarizing structure comprises a CoPd layer that is adjacent to the base electrode layer, a CoFeB polarizing layer, and a MgO spacer layer that is adjacent to the laminated magnetic free layer.
11. The spin transfer torque magnetic junction of claim 8 wherein the magnetic reference layer structure comprises a pinned layer comprising CoFeB that is disposed adjacent the insulative barrier layer, and a pair of antiferromagnetically coupled pinning layers, each having magnetic anisotropy perpendicular to the substrate plane.
12. The spin transfer torque magnetic junction of claim 11 wherein the magnetic reference layer structure further comprises an outer layer comprising an alloy of manganese that is disposed adjacent to the top electrode layer.
13. The spin transfer torque magnetic junction of claim 11 wherein the insulative barrier layer comprises MgO having a thickness in the range of 8 Angstroms to 20 Angstroms.
14. The spin transfer torque magnetic junction of claim 11 wherein the laminated magnetic free layer comprises no more than four sublayers of CoFeB.
15. A spin transfer torque magnetic junction comprising:
- an electrically conductive base electrode defining a substrate plane;
- a magnetic reference layer structure disposed on the base electrode and having magnetic anisotropy perpendicular to the substrate plane;
- an insulative barrier layer disposed on the laminated magnetic reference layer;
- a laminated magnetic free layer disposed on the insulative barrier layer and having magnetic anisotropy perpendicular to the substrate plane;
- a current polarizing structure disposed on the laminated magnetic free layer and having magnetic anisotropy parallel to the substrate plane;
- an electrically conductive top electrode disposed on the current polarizing structure;
- wherein the laminated magnetic free layer comprises first, second, and third sublayers, each comprising a ferromagnetic material selected from the group consisting of CoFeB, CoPt, FePt, and CoPd; and
- wherein the first and second sublayers are separated by a first dusting layer, and the second and third sublayers are separated by a second dusting layer.
16. The spin transfer torque magnetic junction of claim 15 wherein each of the first and second dusting layers comprises tantalum with a thickness in the range of 1 Angstroms to 7 Angstroms.
17. The spin transfer torque magnetic junction of claim 15 wherein the current polarizing structure comprises a CoPd layer that is adjacent to the top electrode layer, a CoFeB polarizing layer, and a MgO spacer layer that is adjacent to the laminated magnetic free layer.
18. The spin transfer torque magnetic junction of claim 15 wherein the magnetic reference layer structure comprises a pinned layer comprising CoFeB that is disposed adjacent the insulative barrier layer, and a pair of antiferromagnetically coupled pinning layers, each having magnetic anisotropy perpendicular to the substrate plane.
19. The spin transfer torque magnetic junction of claim 18 wherein the magnetic reference layer structure further comprises an outer layer comprising an alloy of manganese that is disposed adjacent to the base electrode layer.
20. The spin transfer torque magnetic junction of claim 15 wherein the insulative barrier layer comprises MgO having a thickness in the range of 8 Angstroms to 20 Angstroms.
Type: Application
Filed: Jan 12, 2016
Publication Date: Jun 9, 2016
Patent Grant number: 9705072
Inventors: Shaoping Li (San Ramon, CA), Gerardo A. Bertero (Redwood City, CA), Yuankai Zheng (Fremont, CA), Qunwen Leng (Palo Alto, CA), Shihai He (Fremont, CA), Yunfei Ding (Fremont, CA), Ming Mao (Dublin, CA), Abhinandan Chougule (Gilroy, CA), Daniel K. Lottis (Sunnyvale, CA)
Application Number: 14/993,127